The purpose of this project is to study the immunological effects of Cryosurgery on Moloney murine sarcoma virus (M-MuSV)-induced tumors in mice. The effect of cryosurgery was compared with that of conventional surgery in terms of the following aspects: tumor growth, cumulative mortality, proliferative response of immune lymphocytes, cell-mediated cytotoxicity and humoral antibody formation against tumor cells. Cryosurgery enhanced the immune response of host animals and reduced the tumor incidence and the death of sarcoma invasion. The effect of adaptive transfer of lymphocytes to M-MuSV-induced tumor-bearing mice was studied in terms of the tumor growth. Spleen cells from mice 12 days after virus-inoculation enhanced the tumor growth if they were transferred after tumor development, whereas the same lymphocytes inhibited tumor development if they were transferred 7 days before virus inoculation. Furthermore, these enhancing and inhibiting effects were antigenically specific and mediated by thymus-dependent cell population. The kinetics of these two paradoxical effects were investigated.